two_sum.cpp

/*
Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].
*/

/*
Solution:
use hash map<key, value>, and assign value by map[key] = value
*/

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> mapping;
        vector<int> result;
        // create a hash map to store the index of each entry in nums
        for (int i = 0; i < nums.size(); i++) {
            mapping[nums[i]] = i;
        }
        for (int i = 0; i < nums.size(); i++) {
            const int gap = target - nums[i]; // the target element
            if (mapping.find(gap) != mapping.end() && mapping[gap] > i) {
                result.push_back(i);
                result.push_back(mapping[gap]);
                break;
            }
        }
        return result;
    }
};